1. Field of the Invention
This invention relates to an automatic level controller for audio amplifiers used in transceivers, tape recorders and like audio equipments.
2. Description of the Prior Art
A conventional automatic controller generally used in transceivers and the like will be described with reference to FIGS. 1 and 4 before describing the present invention in detail.
Referring to FIG. 1, such a known automatic level controller includes audio amplifying circuits 3 and 5, low-frequency transformers 4 and 6, and a transmitting circuit 7 including modulating means. The audio amplifying circuit 3 is connected at the input terminal 31 thereof to a microphone 1 through an input resistor 2, and at the output terminal thereof to the other audio amplifying circuit 5 through the transformer 4. The output of the amplifying circuit 5 is coupled to the primary winding of the transformer 6. The transmitting circuit 7 has an output terminal 71. A diode 20 is connected at the cathode thereof to a power source +B through the secondary winding 62 of the low-frequency or modulation transformer 6 and to an input terminal of the transmitting circuit 7. The anode of the diode 20 is connected to the power source +B through a resistor 21 and is grounded through another resistor 22. Further, the anode of the diode 20 is connected to the base of a transistor 14 through a capacitor 23, a rectifying circuit 24 consisting of diodes 241 and 242, and a time constant circuit 25 consisting of a resistor 251 and a capacitor 252. The diode 20 is normally reverse biased by the resistors 21 and 22. The transistor 14 functions to control the audio level of an audio signal applied to the audio amplifying circuit 3, and the collector of the transistor 14 is connected to the input terminal 31 of the audio ampifying circuit 3 to which the resistor 2 is connected, while the emitter thereof is directly grounded.
In operation, an audio signal applied to the microphone 1 is amplified by the audio amplifying circuits 3 and 5 and then passes through the modulation transformer 6 to be applied to the transmitting circuit 7. The signal appearing across the secondary winding 62 of the modulation transformer 6 provides the cathode potential of the diode 20, but the level of this signal varies depending on the magnitude of the audio input signal, whereas the anode potential of the diode 20 is determined by two dividing resistors 21 and 22. When the input signal has a large amplitude large enough to provides a low cathode potential of the diode 20 which is lower than the anode potential by such an extent enabling to turn the diode 20 on, the diode 20 becomes turned on, and this signal passes through the diode 20 to appear across the voltage-dividing resistor 22 as a pulse signal. The maximum value of the amplitude of this pulse signal appearing across the resistor 22 is determined by the d.c. terminal voltage of the resistor 22 and is equal to the d.c. terminal voltage. Therefore, the maximum value of the amplitude of the pulse signal appearing across the resistor 22 can be varied by varying the d.c. terminal voltage of the resistor 22. This pulse signal is rectified by the rectifying circuit 24, and this rectified signal is applied through the time constant circuit 25 to the base of the audio signal level control transistor 14. The transistor 14 is forward biased by this rectified signal and controls the audio level by dividing the audio signal output of the microphone 1 by the internal impedance of the transistor 14 and resistor 2. The curve B shown in FIG. 4 represents the audio level control characteristic of the conventional level controller shown in FIG. 1. The curve A shown in FIG. 4 represents the characteristic observed when any level controller is not provided.
In the conventional level controller having a structure as above described, the diode 20 is turned on only when the amplitude of the signal appearing across the secondary winding 62 of the modulation transformer 6 exceeds a predetermined level (in this case, this predetermined level is the anode potential level of the diode 20 and the signal level appearing at the secondary winding 62 should be lower than the anode potential), and the signal passing through the diode 20 is rectified to provide the control signal for the audio level control transistor 14. However, this conventional level controller has various defects as enumerated below. Firstly, the audio level control range is quite narrow. That is, the controllable range of the conventional level controller is only 30 dB as apparent from the characteristic curve B shown in FIG. 4. Secondly, the audio level control starts at a point B.sub.o which is earlier than desired as apparent also from the characteristic curve B shown in FIG. 4. That is, this audio level control starting point B.sub.o is considerably lower than the 100 percent modulation line or clipping level shown in FIG. 4. As a result, the control effect of the conventional audio level controller is quite gradual and insufficient. These defects are attributable to the fact that the control signal level applied to the audio level control transistor 14 is low.
This audio level control starting point B.sub.o can be further delayed and shifted toward the clipping level shown in FIG. 4 when the d.c. terminal voltage of the resistor 22 in FIG. 1 is reduced for increasing reverse bias voltage for the diode 20. However, this results in further narrowing of the audio level controllable range. It is therefore undesirable to excessively shift and delay the audio level control starting point B.sub.o. On the other hand, when the d.c. terminal voltage of the resistor 22 is increased for reducing reverse bias voltage for the diode 20, the magnitude of the control signal passing through the diode 20 can be increased so that a satisfactorily effective audio level control can be achieved with this control signal of increased magnitude. However, this results in an undesirable situation such that the audio level control starts earlier than the point B.sub.o shown in FIG. 4, that is, the audio level control is initiated at a position which is considerably lower than the clipping level for the input signal. This audio level control starting point B.sub.o is desirably as close to the clipping level as possible.
Thirdly, in the conventional level controller, due to variations of the power source voltage the d.c. terminal voltage of the resistor 22 necessarily varies, hence variations of the audio level control range of the gain control signal appearing across the resistor 22 occur. This defect is attributable to the fact that the audio level control range of the gain control signal is limited by the d.c. terminal voltage of the resistor 22.